Coupled active transport of Na and Cl across forestomach epithelium.

CHIEN, WAN-Ju, AND C. E. Stevens. Coupled active transport of Na and Cl across forestomach epithelium. Am. J. Physiol. 223(4) : 9971003. 1972.-Bidirectional fluxes of Na and Cl across the isolated, short-circuited bovine rumen epithelium were measured by both doubleand quadruple-isotope-labeling techniques. Both Na and Cl were actively transported in the direction of lumen to blood, but since Cl was transported at approximately twice the rate of Na, the algebraic sum of their ion currents did not account for the recorded short-circuit current or even the sign (blood side positive) of the transepithelial electrical potential. Ion replacement experiments indicated that active transport of K or multivalent, inorganic ions could not account for this discrepancy. Removal of Cl from the bathing solution markedly reduced active Na transport to levels which could entirely account for the short-circuit current. Alternatively, the removal of the Na from these solutions almost completely inhibited the short-circuit current and markedly inhibited the active transport of Cl. Replacement of the bicarbonate buffer system with tricine significantly reduced active transport of both Na and Cl. Results indicated that 1) a substantial, active transport of at least three ions was required to explai,n the observed short-circuit current, 2) the additional ion(s) were HCOs and possibly H, and 3) the active transport of these ions was coupled to a significant degree.